A prominent hacker set to be sentenced in federal court this week for breaking into numerous computer systems worldwide has provided a trove of information to authorities, allowing them to disrupt at least 300 cyberattacks on targets that included the U.S. military, Congress, courts, NASA and private companies, according to a newly filed government court document.

The hacker, Hector Xavier Monsegur, also helped authorities dismantle an aggressive cell of the hacking collective Anonymous, leading to the arrest of eight of its members in Europe and the United States, including Jeremy Hammond, who the FBI said was its top "cybercriminal target," the document said. Hammond is serving a 10-year prison term.

The court filing was prepared by prosecutors in New York City who are asking for leniency for Monsegur because of his "extraordinary cooperation." He is to be sentenced Tuesday in U.S. District Court on hacking conspiracy and other charges that could result in a long prison term.

It has been known since 2012 that Monsegur, who was arrested in 2011, was acting as a government mole in the shadowy world of computer hacking, but the memorandum submitted late Friday reveals for the first time the extent of his assistance and what the government perceives its value to be.

Monsegur's whereabouts have been shrouded in mystery. Since his cooperation with the authorities became known, he has been vilified online by supporters of Anonymous, of which he was a member. The memo, meanwhile, said the government became so concerned about his safety that it relocated him and some members of his family. "Monsegur repeatedly was approached on the street and threatened or menaced about his cooperation once it became publicly known," said the memo, which was filed by the office of Preet Bharara, the U.S. attorney in Manhattan.

Born in 1983, Monsegur moved to a housing project on the Lower East Side of Manhattan at a young age, where he lived with his grandmother after his father and aunt were arrested for selling heroin. He became involved with hacking groups in the late 1990s, drawn, he has indicated, to the groups' antigovernment philosophies.

The memo said that when Monsegur, who used the Internet alias Sabu, was first approached by FBI agents in June 2011 and questioned about his online activities, he admitted to criminal conduct and immediately agreed to cooperate with law enforcement.

Neither Bharara's office nor a lawyer for Monsegur would comment about the memo.